The present invention relates to semiconductor device assembly, and more particularly, to a method and apparatus for monitoring free air ball (FAB) formation during wire bonding.
A semiconductor die is a small integrated circuit formed on a semiconductor wafer. The die is cut from the wafer and then attached to a substrate or semiconductor chip carrier. Bonding pads on the semiconductor die are electrically connected to electrical contacts on the carrier (also known as leads or lead fingers) using bond wires with wire bonding equipment. Wire bonding is a solid phase process that uses a combination of heat, pressure and ultrasonic energy to form a connection between a bond wire and the bonding pads and carrier leads.
In semiconductor device assembly of wire bond type devices, a wire is fed into a capillary of a wire bonding machine. Typically, a first, ball bond is used to attach a tip of the wire to a bonding pad on the die. The ball bond is formed by applying a high voltage electrical charge to the tip of the wire, which melts the tip such that a ball is formed at the tip. The ball is then welded to the die bond pad. The wire is then moved to one of the leads of the carrier and a second bond is formed to attach the other end of the wire to the lead of the chip carrier.
The ball formation process is achieved by ionization of an air gap by a process called electronic flame off (EFO). In EFO, electric heating discharge occurs between two electrodes: one electrode is the wire, typically Copper or Gold (anode) and the other electrode is a flat plate (cathode). The heat produced by an EFO probe during the discharge causes the wire electrode to melt and surface tension of the molten metal causes the metal to roll up into a ball shape. When a sufficient amount of the metal (wire) has melted, the discharge is terminated and the molten ball is allowed to solidify. The resulting ball is known as a free air ball (FAB). A suitable reducing agent, such as a forming gas (most commonly a mixture of hydrogen and nitrogen in an appropriate concentration) is used to minimize oxidation of the FAB, which is especially important if the wire is made of Copper.
FABS may suffer from defects and inconsistencies related to shape, size, roundness, roughness and brightness that arise due to variations in EFO parameters, such as the distance between the EFO probe and wire, rate of discharge of the forming gases, and timing of discharge of the forming gases. Incorrectly set EFO parameters result in bonding failures including formation of ‘flower’ balls, ‘golf’ balls, and bonding pad damage. These defects can play an important role in the overall IC production yield and cost. Defective FAB profiles can lead to defects or lower quality, which can cause a device to be rejected during post-production quality checks. Moreover, if a wire bonder produces defective FABS, the entire production batch may be rejected leading to high production costs and low yield. Therefore, monitoring and inspection of the FAB formation during the wire bonding process is desired to save time and cost.
In light of the foregoing, there is a need for monitoring FAB formation in a wire bonding process.